Polymerization of inorganic element-containing monomers using plasma

ABSTRACT

An ionized gas plasma is established in an electrical field in contact with a non-vapor volume monomer (liquid and/or solid). The plasma causes polymerization of the monomers which are of the phosphazene or carborane type.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of our copending patentapplications, Ser. No. 934,855, filed Aug. 18, 1978, entitled "A Methodof Plasma Initiated Polymerization", now U.S. Pat. No. 4,212,719 and ofits parent application, Ser. No. 882,124, filed Feb. 28, 1978, of thesame title, now abandoned.

BACKGROUND OF THE INVENTION

Application of an electric field to a gas, generally of a vacuum, toform a plasma of gas ions is a known technique for use in thepolymerization of a variety of monomers. This technique has beendescribed, for example, in Luster U.S. Pat. No. 2,257,177, Tobin U.S.Pat. No. 3,287,242, and a variety of other patents.

The class of phosphazene and carborane polymers are of potentialpractical importance because of their high temperature stability. In thepast, techniques other than plasma polymerization have been employed toform polymers from phosphazene monomers. For example, heat, gamma rays,x-rays, and high energy electron excitation have been employed for thispurpose. These techniques are not desirable because of the relativelyhigh energy and power consumption required for polymerization. Inaddition, the yields by such polymerization techniques are relativelylow. Carborane monomers have been copolymerized with other monomers suchas organsiloxanes by the technique of condensation polymerization.However, there is no technique in the prior art which permitshomopolymerization.

SUMMARY OF THE INVENTION AND OBJECTS

In accordance with the present invention, polymerization of a non-vaporphase (liquid and/or solid) monomer is accomplished by polymerization asthe result of contact with an ionized gas plasma. The monomers are ofthe phosphazene or carborane form both of which are crystalline solidsunder ambient conditions and in their structural formulae include theinorganic elements phosphorus or boron in the repeating units of thepolymer. Such polymers have exceptional high temperature stability.

It is an object of the invention to provide a versatile ionized gasplasma polymerization technique of low power input for thepolymerization of phosphazene or carborane monomers.

It is a further object of the invention to provide a technique of theforegoing type capable of homopolymerizing carborane.

It is another object of the invention to provide a method of theforegoing type capable of relatively high polymer yields.

Other objects and features of the invention will be apparent from thefollowing description of the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principle of the present invention is to polymerize a specific classof two related polymers by the use of a non-equilibrium ionized gasplasma. The formation of the ionized gas plasma (herein plasma) is setout in detail in our parent applications. Such formation may beaccomplished by any known technique, for example, see J. R. Hollahan andA. T. Bell, eds., "Techniques and Applications of Plasma Chemistry",Wiley, N.Y., 1974 and M. Shen, ed. "Plasma Chemistry of Polymers",Dekker, N.Y., 1976. In one technique, an ionizable gas is containedunder vacuum between parallel plate electrodes connected to aradiofrequency generator which is sold by International PlasmaCorporation under the designation "Model 3001". The plasma can becreated with such parallel plates either external or internal to thevacuum chamber. In another technique, an external induction coil createsan electric field which produces the plasma. In yet another technique,oppositely charged electrode points are placed directly into the vacuumchamber in spaced apart relationship to create the plasma. The presentinvention is applicable to any plasma formed by these techniques or anyother one in which an electric field creates a path of electricalconduction within the gas phase.

The operating parameters for the plasma vary from monomer to monomer. Ingeneral, it is preferable to employ reduced gas pressure to form a glowdischarge by electron liberation which causes ionization in the gasphase. The power requirements of the system are relatively low, on theorder of 10 to 1000 watts. In addition, the energy of the plasma is atan order of magnitude lower than that of so-called high energypolymerization techniques such as electron beams, gamma rays, andx-rays. Thus, for example, the energy of electrons in the plasma issubstantially less than 100 eV and typically on the order of 1-10 eV onthe average.

It has been found that plasma contact on the order of 10 minutes to 10hours is sufficient for plasma polymerization to relatively highconversion. In an optional subsequent procedure, the polymer formedduring the plasma polymerization may be heated to an elevatedtemperature during which polymerization continues to a higher degree ofconversion. This step, termed "postpolymerization", should be performedat a temperature sufficient to melt the solid crystalline monomer. Forexample, for hexochlorocyclotriphosphazene monomers, thispostpolymerization may be carried out on the order of 120° to 140° C.,above the melting point of formed polymers. Suitable postpolymerizationtimes may range from 1 hour to 24 hours or more. Postpolymerization isless satisfactory for carborane polymers formed by plasmapolymerization. This is because such polymers have relatively highmelting points, e.g., 290° C.

Where a plasma is created in a chamber including a monomer gas at apressure below atmospheric pressure, the plasma is formed when theinterelectrode potential exceeds a threshhold value which is sufficientto ionize or "breakdown" the gas. This is a function of the compositionof the gas, its pressure and the distance between the electrodes. Afterbreakdown, the gas is conductive and a stable plasma may be maintainedover a wide range of currents. Although the exact composition of theplasma is not known, it includes electrons, ions, free radicals, andother reactive species.

The present polymerization technique is applicable to ring-type monomersof the phosphazene or carborane class. Such compounds are related inthat they include inorganic elements (phosphorus or boron) as part of arepeating unit of the polymer. Also, they are in the solid crystallineform at room temperature and are characterized by high temperaturestability.

THE PHOSPHAZENE EMBODIMENT

The term "phosphazene monomer" refers to a ring compound including##STR1## repeating units in a ring. The term "monomer" includespolymerizable trimers, tetramers and pentamers. One particularlysuitable phosphazene monomer is the dichlorophosphazene trimer termed"hexachlorocyclotriphosphazene". That product may be structurallyrepresented by the following formula where n=3. ##STR2##

In the monomer, other halogen groups (fluoride and bromide) may besubstituted for the chloride group. In addition, organofunctional groupssuch as OR, NHR, NR₂, or R may be substituted, wherein R is an aliphaticor aromatic group. Such substitution may occur prior or subsequent topolymerization. Thus, the formula (1) monomer may be plasma polymerizedfollowed by substitution of the organic radicals for the halogens in thepolymer by conventional chemical techniques. Alternatively, suchsubstitution may be made in the monomer prior to polymerization.

A large and expanding number of poly(organophosphazenes) have beensynthesized by the reaction of poly(dichlorophosphazene) with alkoxides,arlyoxides, or amines. Poly(organophosphazenes) of this type includepoly[bis(trifluoroethoxy)phosphazene],poly[bis(4-methylphenoxy)phosphazene], and poly[bis-(4-methoxanilino)phosphazene].

The phosphazene polymer is defined in the manner of the foregoingmonomer with the exception that the number of repeating units (n) is atleast 9. Thus, the preferred phosphazene polymer product may be definedby the following generalized structural formula: ##STR3## wherein n=thenumber of repeating units, and y and z are selected from the groupconsisting of chloride, bromide, flouride, and OR, NHR, NR₂, or R,wherein R is an aliphatic or aromatic group, and mixtures thereof.

Plasma polymerization of phosphazenes without post-polymerizationproduce relatively high conversions of monomer to polymer at times of 10to 30 minutes. For example, conversion percentages of from 15 to 60% ormore are obtainable. Postpolymerization at elevated temperatures doesnot appear to increase such conversion percentages.

The phosphazene monomers may be plasma polymerized as homopolymers orcopolymerized with other phosphazene polymers or with totally differentmonomers. The polymers formed only from phosphazene monomers arecharacterized by excellent high temperature stability. Also, they arecharacterized by a relatively high viscosity at elevated temperatures.Other monomers such as 1,3,5-triazine, s-triazoborane and1-H-1,2,4-triazole may be similarly polymerized.

CARBORANES

Carborane monomers are generally crystalline solids at room temperature.Three specific structural formula of such monomers are as follows:

C₁₋₂ B_(n) H_(n+2) for closo (cage) structures;

C₁₋₄ B_(n) H_(n+4) for nido (nest) structures; and

C₁₋₆ B_(n) H_(n+6) for anachno (web) structures;

where n is generally in the range of 3-10.

Suitable carboranes are one or more monomers of the foregoing structuralformula.

It has been found that carboranes may be homopolymerized by theforegoing technique of plasma polymerization of phosphazene monomer. Inaddition, two or more carboranes may be copolymerized. Furthermore,other monomers such as organosilanes, including dimethyldichlorosilane,dimethylsilanediol, phenylmethyldichlorosilane, andtrifluoromethyldichlorosilane, may be copolymerized together with thecarboranes.

Suitable nido-carboranes include CB₅ H₉, C₂ B₄ H₈, C₃ B₃ H₇, C₄ B₂ H₆,and the like. Suitable closo-carboranes structures include 1,5-C₂ B₃ H₅,1,6-C₂ B₄ H₆, 2,4-C₂ B₄ H₇, C₂ B₆ H₈, C₂ B₇ H₉, 1,10-C₂ B₈ H₁₀, C₂ B₉H₁₁, 1,7-C₂ B₁₀ H₁₂, and ortho-1,10-C₂ B₁₀ H₁₂. Particularly effectivepolymers are produced with the last named monomer, commonly referred toas o-carborane.

The carborane homopolymer produced from the foregoing ring compounds arecharacterized by structural formula (3) with the exception that thenumber of repeating units (n) is at least 9.

Relatively high yields are obtained by plasma polymerization ofcarboranes. For example, conversions for o-carborane are expected to befrom 25 to 50% or more.

Poly(o-carborane) polymer is characterized by high temperaturestability. For example, it does not decompose at temperatures as high as800° C. It is characterized by insolubility in benzene to form a whitefilm which is insoluble in most organic solvents.

The conditions of plasma polymerization are generally the same as thoseof plasma polymerization of the phosphazenes. Thus, the plasma may beformed in the foregoing manner. Also, suitable plasma contact times areon the order of 10 minutes to an hour. Because of their high meltingpoints, postpolymerization of the polymers in the molten state are notdeemed to be practical except for highly specialized end uses.

It is noted that for both the phosphazenes and carboranes, there isconsiderable flexibility with respect to the degree of cross-linkingwhich may be obtained in the resultant product. Thus, depending upon thedesired end use, the product may be relatively straight chained orinclude a high degree of cross-linking depending upon the conditions andthe particular monomers employed.

A further disclosure of the nature of the present invention is providedby the following specific examples of its practice. It should beunderstood that the data disclosed serve only as examples and are notintended to limit the scope of the invention.

EXAMPLE 1

The above technique was employed for the polymerization ofhexachlorocyclotriphosphazene in accordance with the followingconditions. This solid crystalline monomer was sealed in a glass tubeafter degassing at 10⁻³ to 10⁻⁴ torr., and subsequently frozen in liquidnitrogen. The glass tube was then inserted between a pair of externalparallel plate electrodes connected to an International PlasmaCorporation Model 3001 radiofrequency generator which operates at 13.56MHz frequency and delivers up to 150 watts power. A glow discharge wasthen generated for 30 minutes at 50-120 watts at room temperature. Thevapor pressure inside the tube was controlled through a stopcock tosustain the plasma. The surface of exposure to the plasma, a portion ofthe solid monomer surface became molten and increased viscosity duringplasma contact. The plasma was then terminated and the product wasextracted in warm solvent (1,1',2,2'-tetrachloroethane) with stirringfor about 1 to 5 hours. The resulting polymer was cross-linked andexhibited a rubber-like property. The polymer yield or conversion ratiowas on the order of 10%.

EXAMPLE 2

The monomer of Example 1 was polymerized by the technique of thatExample except for the following differences. The phosphazene wasexposed to the plasma for 3 hours (at 80 watts for the first hour and at120 watts for the following 2 hours). Then the reaction mixture wasallowed to postpolymerize in an oil bath maintained at 210° C. for 15hours. By way of comparison, the same amount of a phosphazene referencemonomer which had not been exposed to the plasma was sealed under vacuumand left standing at the same temperature.

The plasma contacted reaction system exhibited a substantial increase inviscosity (the consistency at 210° C. of a syrup in comparison to thereference which exhibited only a slight increase in viscosity (aconsistency at 210° C. of water at room temperature). The polymer didnot dissolve in toluene.

The yield of polymer which had been exposed to plasma was about 60%conversion. In contrast, the yield of the reference was less than 5%.

EXAMPLE 3

The o-carborane was polymerized in accordance with the conditions ofExample 1.

The glass surface of the tube was gradually covered by a polymer film ofincreasing thickness as plasma proceeded. The polymer yield was on theorder of 10% conversion. The resulting polymer film was white colored,thin and insoluble in most organic solvents. It exhibited excellentthermal stability.

EXAMPLE 4

The procedure of the preceding Example was performed with the exceptionthat the plasma conditions were 50 watts of power at 25 minute duration.In addition, the polymer was postpolymerized at 80° C. for 20 hours.

The resulting polymer conversion was about 25%. Under thermal analysis,the product did not decompose up to 800° C. As with Example 3, theproduct was insoluble in benzene and most organic solvents and formsinto a white film.

What is claimed is:
 1. A method for the polymerization of a phosphazenemonomer comprising establishing and maintaining a contained zone ofionized gas plasma at an energy of less than 100 eV by applying anelectric field, said gas plasma being in contact with the surface of anon-vapor volume of said phosphazene monomer to cause it to polymerize.2. The method of claim 1 in which the structural formula of saidphosphazene polymer product is the following: ##STR4## wherein n=thenumber of repeating units, and Y and Z are selected from the groupconsisting of chloride, bromide, fluoride, OR, NHR, NR₂, and R, whereinR is an aliphatic or aromatic group, and mixtures thereof.
 3. The methodof claim 1 in which said phosphazene monomer is in crystalline solidform during plasma contact.
 4. The method of claim 1 in which saidphosphazene monomer comprises dichlorophosphazene.